The invention relates to the field of methods for processing a radar signal in land/sea detection mode.
A mechanically or electronically scanned pulse radar, for example carried by a sea surveillance airplane, is said to be pulsed when it periodically emits an electromagnetic pulse.
The signal received in return by the radar is first converted into a digital signal by passing through an analog pretreatment stage and an analog/digital conversion stage.
Then, as shown in FIG. 1, the resulting digital signal, S(t), depending on the time t, is next sampled using a two-dimensional sampling temporal map. The signal is sampled both using a short time, corresponding to a distance dimension d, and using a long time, corresponding to a recurrence dimension rec. The recurrence dimension is equivalent to azimuth angular information φ. Below, we will refer indifferently to recurrence or angle for this temporal dimension.
A distance sampling interval, or range bin, corresponds to the distance resolution of the radar. Likewise, a recurrence sampling interval, or recurrence bin, corresponds to the angular resolution of the radar.
After the sampling step, which makes it possible to obtain raw distance/recurrence samples Ê(d,rec), the processing of the signal continues with a step for compensating movements of the airplane, so as to obtain distance/recurrence samples. Each distance/recurrence sample E(d,rec) is associated with a cell of the temporal map, identified by a distance bin d and a recurrence bin rec.
Then, in the case of non-coherent detection processing, the power of each distance/recurrence sample is computed, then associated with the corresponding cell so as to obtain a distance/recurrence representation, called temporal representation (RT), of the zone observed by the radar.
Furthermore, in the case of coherent treatment, a rapid Fourier transform RFT may be applied, using the recurrence dimension, to the distance/recurrence samples E(d,rec), so as to sample the signal S(t) using a dimension corresponding to Doppler frequency information Δf, dual for the recurrence dimension. A Doppler frequency sampling interval, or frequency bin, corresponds to the frequency resolution of the radar. Distance/Doppler frequency samples E(d, Δf) are thus obtained.
Then, the power of each distance/Doppler frequency sample E(d, Δf) is computed, and associated with the corresponding cell so as to obtain a distance/Doppler frequency representation, called frequency representation (RF), of the zone observed by the radar.
After these processing steps of the signal, detection processing is applied on the temporal representation RT and/or on the frequency representation RF, to identify targets.
In general, the signal S(t) includes useful echoes, corresponding to targets of interest that one wishes to detect, and stray echoes, in particular made up of the land clutter or sea clutter. Land clutter corresponds to obstacles on the surface of the land (buildings, vegetation, etc.), and sea clutter corresponds to obstacles on the surface of the sea (waves). These obstacles return an echo toward the radar. The land clutter and sea clutter have characteristic and known imprints, in particular frequency imprints.
In a first step of the detection processing, a detection threshold is applied on the used representation (RT or RF) to select only the cells whose power is above this threshold. The samples corresponding to these selected cells constitute detections.
The detection processing is done so as to obtain a constant false alarm rate (meaning a detection that is not actually a target). To that end, the detection threshold is computed dynamically based on the number of detections to which the application of a certain threshold level leads. For example, if the zone to be observed is a sea surface, the detection threshold will be low. Conversely, if the zone to be observed is a land surface, the threshold will be high. Indeed, the land reflectivity coefficient being higher than that of the sea, land generates many stray echoes. These will be considered detections, if the threshold level remains low. The threshold is consequently raised for a land surface.
However, to monitor a coastal zone, the zone observed by the radar, then working in land/sea mode, may include both a land surface and a sea surface.
Yet the land surface constitutes a disruptive element in detecting targets on the surface of the water. Indeed, if the detection threshold is not raised, the stray echoes from the land surface lead to a false alarm rate that increases uncontrollably. Furthermore, the number of detections to be monitored increases the computing load. Lastly, the high number of false alarms causes a confusing tactical situation.
Thus, near the coast, the detection threshold is raised. It is then no longer optimized for detecting small targets on the surface of the water, small targets being characterized by a low Surface Equivalent Radar (SER). Consequently, the likelihood of detecting targets with a low SER is downgraded in land/sea detection mode.
The invention therefore aims to resolve this problem.